The separation of a mixture into its components ha been performed by any number of techniques in the past. Such separation techniques include distillation, extraction, chilling/filtration, and more recently, reverse osmosis through a membrane which in general depends upon molecular type differences between components in a stream, perstraction and pervaporation which rely upon differences in molecular type and solubility in the membrane, and ultrafiltration which, in general, relies upon size differences to effect separation.
Distillation is typically employed when the boiling points of the components in the solution, mixture or melt are sufficiently different so that a relatively clean separation of the components can be achieved. Materials of different molecular type but similar boiling point are not readily separable by distillation.
Extraction effects the separation of components in a mixture by utilizing the differences in solubility of the components in some added, easily separated/recovered solvent. For example, aromatic hydrocarbons are separated from generally similar boiling point aliphatic hydrocarbon by taking advantage of the higher solubility of aromatics in an extraction solvent such as phenol, furfural or NMP. The extraction solvent selective separttes the aromatics from the mixture; the aromatics themselves are easily separated from the extraction solvent.
Crystallization from a solution or melt followed by filtration is a well established separation technique. It has found application in oil dewaxing. In general, the oil to be dewaxed with or without added dilution solvent is chilled to a temperature at which the wax crystallizes producing a wax/oil slurry. This slurry is subsequently filtered to yield a dewaxed oil. Thus, large wax crystal size is desirable for efficient filtration. This chilling is usually to a temperature well below the desired pour point of the oil and is called the filter temperature. Thus, to achieve a pour point of about -15.degree. C., one would have to chill the oil to a temperature lower than -15.degree. C.
Reverse osmosis separates components by molecular type and employs applied pressure to overcome the osmotic pressure and selectively permeate one component through a non-porous membrane. Perstraction and pervaporation separate components on the bases of molecular type and rely upon the selective dissolution of one component in the membrane and migration of the component across the membrane in response to a concentration gradient. Ultrafiltration separates molecules on the basis of difference in molecule particle size.
In the present invention it has been discovered that in effecting an ultrafiltration separation of components of different melting/solidification point temperature it is not necessary, and in fact, is detrimental to ultrafilterability to cool the mixture to a temperature at or below the phase transition temperature of the higher melting component to be separated. The phase transition temperature, for the purposes of this application and the appended claims, is defined as the temperature at which macroscopic crystals are formed, and operating temperature is that at which the component to be removed is just in the crystallite or submicro crystal form. Thus, operation must be at just above the phase transition temperature.